Painted his shield to red , and have a little problem with his cloack , it cannot be red , green or black, because those colors I've already used ...
Why not? People sometimes deliberately chose colours that went together as part of a personal livery of sorts, but it could be simply because they liked a particular colour just as you might today
If you wish to use 'the same' colour on two parts of a figure and want to avoid that problem where things look too alike you just need to start from a different paint or mix the colour differently. So either vary the hue slightly, e.g. one red more orangey than the other, or make one brighter and the other more dull. As well as either of these possibilities a good way to show the difference between a painted surface and a dyed fabric is to make one slightly glossy and the other relatively matt.
And I want some trips about historical reliability of the cloack's color and the rest of the figure
There's now a fair bit online about historical dyes and it's worth doing a bit of your own research to get an idea of what was possible, affordable or expensive, common or rare. With subjects from specific regions it can be important to know what was easily available at a given period in history, in the far north of Europe for example they had access to dyestuffs they didn't use around the Mediterranean.
In terms of a dyed colour black was surprisingly expensive because it would have involved dyeing a piece more than once (twice or three times). But of course for wool there is the possibility of black wool* having been used so no dying necessary.
And there were not any good greens available for a lot of history, so it was primarily achieved by dying with both blue and yellow, so again relatively expensive compared to say a russet, dull yellow, paler blues and anything in its natural colour.
*Very dark brown/near-black in colour.
Einion
